The present invention relates to a dosing device and method for dosing gas in discrete volumes from a pressurized gas supply. The device/method is designed to make the dosing precise, repeatable, and safe. The dosing device/method may be utilized in the delivery of e.g. sulphur hexa fluoride (SF.sub.6) to the lungs of a patient for determining lung functional residual volume (FRC) or can be utilized with other gases for other diagnostic purposes. The device/method can also be used in inhaled nitric oxide (INO) therapy.
The gases dosed by the present invention are supplied in synchronism with the inspiration of the patient. State of the art dosing devices are based generally on gas flow measurement and control. See European patent publication 659,445 and U.S. patent application Ser. No. 5,918,596, filed Jul. 6, 1999 by the present inventor. However, these flow control systems tend to be expensive to manufacture due to the cost of flow sensors and the applicable control devices.
An alternative method is presented in European patent publication 806,216. This publication describes a constant volume dosing device comprising a charging valve, a dosing chamber, and a discharge valve. The charging valve is used to pressurize the dosing chamber with the constant pressure of the pressurized gas source. The discharge valve is used to relieve the pressure from the dosing chamber. Two different operational modes are suggested. In a first mode, the discharge valve is kept closed when the charging valve is opened to charge the chamber and then the reverse operation occurs. In a second mode, the charging valve is kept continuously open while the discharge valve is pulsed.
The device, however, does not fulfill existing safety requirements, especially when delivering gases, such as NO, which while therapeutic in proper doses are toxic in higher amounts. And, interruption of gas delivery may have immediate and disastrous effects on the patient. No means are presented by which one can determine the correct operation of the valves. This may lead to hidden failure of one or both valves and to severe overdosing of the patient. Or, if either of the valves is unable to open, underdosing without any notice will occur.
Another safety concern may arise when the dosing device is attached into the patient airway tube used to administer breathing gases to the patient, as shown in the European patent publication. The dosing device is supplied with high pressure from a source of dosing gas through a high pressure supply line. If a disconnection failure occurs in this high pressure supply line, an uncontrolled exhaust of gas can occur near the patient. The hazardousness of such an exhaust depends on the properties of the gas being administered. This hazard may be exacerbated by the fact that the high pressure supply line and the dosing device, when attached to the airway tube, are typically located in the most crowded working area of an intensive care unit. In addition to the high pressure supply line, also electrical signal lines to operate the valves have to be located in this crowded working environment. These multiple connections may reduce the ergonomics of the dosing system described in the '216 European patent publication.